1. The Field of the Invention
This invention relates generally to communication systems exchanging data between computers over a communication channel. More particularly, the invention relates to improving signal quality of transmitted signals in a noisy communication channel.
2. Present State of the Art
Digital communicating devices employ modulation techniques for transmitting data between a transmitting device such as a computer and a receiving device which also may take the form of a computer. In order to propagate such data between a transmitting computer and a receiving computer, digital information must be transformed into a propagatable format capable of traversing a communication channel. To conform with such a requirement, a transmitting computer employs a transmitting modem for converting digital data into an analog format capable of propagation across the communication channel. Likewise, a receiving computer employs a receiving modem for capturing or receiving the analog signals and converting them back to a digital format readable by the receiving computer.
As transmitting computers and receiving computers may be remotely located one from another, a variety of communication channels or networks are employed for facilitating the routing of the signals between the computers. In a communication system wherein a network is employed for routing between the transmitting computer and receiving computer, a unique communication channel is formed through which the modulated data is transmitted. Such a communication channel provides a unique conduit having inherent ambient noise therein. Therefore, input signals that are transferred from the transmitting computer to the receiving computer acquire the communication channel noise while passing therethrough.
While dedicated communication channels having an exclusive direct path between the transmitting computer and receiving computer may exhibit similar noise characteristics between successive communication sessions, the channel noise on such a communication channel may vary from session to session depending upon dynamic factors such as adjacent cross-talk, ambient noise exposure levels and electronic component aging associated with the coupled transmitters and receivers. While the dynamic communication channel noise envelope assumes a generally small profile, interconnection of the transmitting computer with the receiving computer through a dynamic switching network such as a public switched telephone network (PSTN) may provide a unique physical path for successive communication channels thereby exhibiting drastically dissimilar channel noise characteristics.
In modern communications systems that employ modems, various modulation techniques have arisen to maximize the data rate between a transmitting modem and a receiving modem. To facilitate the transfer of additional data in such a system, advanced modulation techniques have been employed to maximize the dynamic range of signals through a communication channel. Modern modulation techniques utilize changes in transmit signal magnitudes to designate digital words or patterns of bits. For example, quadrature amplitude modulation (QAM) employs both magnitude and phase characteristics of a signal to signify a symbol representing a series of data bits. Therefore, the transfer of signals having a high magnitude level traverse the communication channel with a signal level substantially higher or larger than those signals introduced by the communication channel in the form of channel noise. However, smaller magnitude signals generated by the transmitting modem generally exhibit a magnitude only marginally higher than those magnitudes exhibited by the channel noise. One measure of a signal's quality in a communications system is denoted by a signal-to-noise ratio. Receiving modems rely upon a favorable signal-to-noise ratio for reliably extracting valuable signals from the received composite signal that includes the derogatory effects of channel noise. In many instances, smaller magnitude input signal, may be obscured or incorrectly decoded by the receiving modem due to the marginal difference in signal levels between the low-magnitude input signals and the channel noise as perceived at the receiving modem.
FIG. 1 depicts a prior art approach to equalizing signal levels for evaluation by a receiving modem. In FIG. 1, a transmitting modem 100 generates a series of bands of data represented as input signal 102. Such input signals if pristinely delivered to a receiving modem could be directly interpreted by the receiving modem with unity accuracy. However, as described above, input signal 102 traverses a communication channel before arriving at a receiving modem 108. The traversal of a communication channel 104 combines channel noise 106 with input signal 102. Furthermore, the traversal of input signal 102 through communication channel 104 subjects the various bands to inherently degrading effects which reduce the magnitudes of various bands of the input signal information. As a result, receiving modem 108 receives a received input signal 110 comprised of the degraded input signal mixed with the channel noise. Prior attempts to rectify the composite signal into a more usable and hence reliable form have employed an equalizer 112 for boosting or reduction of the received signal bands to an enhanced magnitude. Such an enhancement procedure is performed by employing a training process upon the initiation of the communication session to determine the derogatory effect of the communication channel on particular bands of information. Such a training process results in an equalization process 114 deriving an equalization curve 116 comprising the boosting or reduction of values used for enhancing the received input signal 110. The equalization process results in an equalized input signal 118 wherein frequency bands received by receiving modem 108 are enhanced prior to the extraction of the signal information from the input signal.
In FIG. 1, equalized input signal 118 depicts the received signal bands in their enhanced state. For example, band 120, 122, 124 and 126 are augmented to approximate the magnitudes exhibited by input signal 102. While such an equalization process provides suitable enhancement for bands having relatively small noise levels, bands coinciding with the channel noise bands do not result in as favorable condition as the other bands having smaller magnitude channel noise components. As depicted in FIG. 1, while band 122 is enhanced to approximate the magnitude of input signal 102 in the corresponding band, the channel noise injected by communication channel 104 is also enhanced as illustrated by enhanced channel noise 128. While the enhancement process improves the magnitude of band 122, the signal-to-noise ratio as evaluated at band 122 has not been improved. Therefore, extraction of the information in band 122 by the receiving modem has not been improved by employing such an equalization process.
Additionally, other prior art approaches to compensating for channel noise have employed a static equalization process wherein a static equalization curve is employed for all communication sessions between a transmitting modem and a receiving modem. While such a static equalization process may marginally suffice for a "standard" communication channel, the dynamic nature of modern communication systems introduce a very dynamic profile of channel noise.
Thus, what is needed is a method and apparatus for improving the signal-to-noise ratio of low-magnitude input signals occurring or coinciding with the channel noise band of a communication channel. Furthermore, what is desired is a method and apparatus for dynamically evaluating the communication noise resident on a communication channel and compensating for such channel noise to improve the signal-to-noise ratio available to a receiving modem in extracting the input signal from the combined input and channel noise signals.